Access to β-Alkylated γ-Functionalized Ketones via Conjugate Additions to Arylideneisoxazol-5-ones and Mo(CO)6-Mediated Reductive Cascade Reactions

1,4-Conjugate addition of ((chloromethyl)sulfonyl)benzenes to arylideneisoxazol-5-ones, followed by one-pot, N-selective trapping in the presence of electrophiles, was investigated. This strategy led to the synthesis of new, stable N-protected isoxazol-5-ones in good yields and high diastereolectivity. The study of the reactivity of obtained products in the presence of the Mo(CO)6/H2O system allowed the development of a cascade reaction leading to novel methyl ketones in high yields and unchanged dr bearing an uncommon chloromethinearylsulfonyl end group.


■ INTRODUCTION
1,4-Conjugate addition of nucleophiles carrying a leaving group (LG) in the α-position like ((chloromethyl)sulfonyl)benzene (PhSO 2 CH 2 Cl) is particularly useful in the development of effective cyclopropanation reactions, 1 while the isolation of the respective Michael adducts has been rarely accomplished. 2 The presence of the LG drives the reactivity of this pro-nucleophile in several other domino reactions as typically in vicarious nucleophilic substitutions (VNS reactions) at electron-deficient arenes 3 or in the formation of oxiranes (Darzens condensation) when combined with carbonyl compounds. 4 In our recent study, we were able to tune the reactivity of ((chloromethyl)sulfonyl)benzene in the addition to carbonyls with the introduction of a further electrophilic cyano group as in 2-acetylbenzonitriles, which competed with chloride displacement of the alkoxide intermediate, leading to the formation of isoindolin-1-ones instead of oxiranes. 5 Isoxazol-5-ones are heterocyclic compounds, which are straightforwardly obtained by condensation of hydroxylamine with readily available β-ketoesters. This class of heterocycles gains increasing interest 6 because of the wide range of biological properties as anti-cancer, 7 anti-microbial, 8 antiobesity, 9 and anti-inflammatory agents 10 or as functional materials in non-linear optical 11 and luminescent probes (see Figure 1 for selected examples). 12 In addition, the rather labile nature of the N−O bond combined with the unique properties of the isoxazole ring enables the synthesis of numerous classes of diverse acyclic and cyclic compounds under several reaction conditions. 6, 13−18 Isoxazol-5-ones are characterized by relatively high acidity at C-4 (pK a 4−6), 6a and the resulting carbanions find a wide use as nucleophiles, 6a−d which can also be used in condensation reactions with aldehydes to generate electrophilic arylideneisoxazol-5-ones (Scheme 1). 6 Michael reactions of arylideneisoxazol-5-ones suffer, however, from a relatively limited scope. 6,14 Tautomerism and scarce stability of the adducts are the main drawbacks (Scheme 1). 6,14 Non-isolated Michael adducts have been directly transformed into acyclic ketones or alkynes by nitrosative cleavage of the N−O bond in the presence of FeSO 4 / NaNO 2 13,14 into diverse heterocyclic compounds using molybdenum reagents 15,16 or as part of domino reactions in the presence of multifunctional nucleophiles, leading to spirocyclic compounds. 17 On the other hand, when a reactive electrophilic component was included at the end of 1,4conjugate addition of malonate diesters to arylideneisoxazol-5ones, the selective N-trapping of the adduct was achieved in high efficiency, preserving the isoxazol-5-one architecture. 18 The obtained products showed high stability and were easily purified by standard techniques, while any attempt to purify or isolate the unprotected Michael adducts led to failures. 18 As part of our research interest in the development of new reactions involving multifunctional electrophiles and nucleophiles, in the present work, we have investigated Michael reactions of arylideneisoxazol-5-ones with ((chloromethyl)sulfonyl)benzenes and the capacity of the obtained adducts to undergo Mo(CO) 6 /H 2 O promoted reductive cleavage of the O−N bond (Scheme 1). After conjugate addition, the enolate intermediate could give intramolecular displacement of the chloride, leading to cyclopropane formation. On the other hand, the N-selective trapping of the enamine form of the adducts by an electrophile E−X should preserve the structure of the chloromethinephenylsulfonyl end group (Scheme 1).

■ RESULTS AND DISCUSSION
Conjugate Additions of ((Chloromethyl)sulfonyl)benzene to Arylideneisoxazol-5-ones and Subsequent Trapping with Electrophiles. Intrigued by this possible dualism (see the scheme of Table 1), in a first set of reactions, we investigated the reactivity of the carbanion of ((chloromethyl)sulfonyl)benzene 1 (PhSO 2 CH 2 Cl) quantitatively generated by reaction with KO t Bu (1 equiv), with 3methyl-4-benzylideneisoxazol-5-ones 2 in anhydrous acetonitrile. Complete conversion was detected at −20°C after 4 h of reaction time by thin-layer chromatography (TLC) ( Table 1, entry 2), while at rt, we observed a series of unknown decomposition products (entry 1). 1 H NMR spectroscopic analysis of the reaction mixture, obtained under the conditions of entry 2, in CD 3 CN was inconclusive because the formation of a precipitate affected the spectra. After the evaporation of the solvent, 1 H NMR analysis in CDCl 3 gave somewhat better indications, highlighting the disappearance of 2, the presence of signals compatible with the protonated Michael adduct of I-3a, and the lack of the cyclopropyl moiety (Table 1, entry 2). However, every attempt to purify the crude by chromatography led to decomposition together with the isolation of the starting materials, probably due to retro-Michael reaction occurring on silica gel. Therefore, we investigated the possibility to obtain stable products by the addition of Scheme 1. General Reactivity of Isoxazol-5-ones and Present Work ACS Omega http://pubs.acs.org/journal/acsodf Article electrophiles E−X at the end of the Michael reaction at −20°C , as previously reported with dimethyl malonate. 18 Our choice focused on di-tert-butyl di-carbonate aiming to a Nselective interception of the I-3a intermediate. Under the conditions of entry 3, the sequential reaction allowed the isolation after chromatography of stable N-Boc-protected 4a, bearing an uncommon chloromethinephenylsulfonyl side chain in good yield and excellent dr >94:6. The use of weaker bases, K 2 CO 3 and Et 3 N, was not effective in promoting the Michael reaction since unreacted starting materials were detected by TLC and 1 H NMR analysis of the crude materials (entries 4 and 5).
Synthetic access to stable, highly functionalized isoxazol-5ones is a very important aim since this class of compounds shows a wide range of biological activities and interesting optical properties (Figure 1). 6−12 Therefore, the scope of the sequential reaction was thoroughly analyzed combining different readily available 1a,5 ((chloromethyl)sulfonyl)benzenes and 3-methyl-4-arylideneisoxazol-5-ones 18 bearing electron-withdrawing and electron-donating groups on the aromatic rings of both the nucleophiles and electrophiles (Table 2). Apart from di-tert-butyl dicarbonate, two other reagents E−X, acetic anhydride and iodomethane, were used in order to investigate if they could lead to products with different substituents on the nitrogen of the heterocyclic ring (Table 2). Based on the data reported in Table 2, the method proved to be effective with all the combinations of substrates, affording in good yields a wide range of new N-protected, stable products 4 in the enamine form, demonstrating the efficiency of the electrophilic trapping strategy also in the presence of alkylating or acylating reagents.
Excellent diastereomeric ratios, up to >99/1 dr were detected in most of the cases. We have not investigated the mechanism to explain this rather general outcome and the few exceptions, which seem to be independent of the substituents and the type of electrophilic trapping reagents. It is likely that the high diastereoselectivity is the result of kinetic control and in a few cases, epimerization occurs to a certain extent. Since we were not able to prepare single crystals of the compounds obtained, the relative configuration was deduced by correlating the experimental and calculated 1 H NMR spectra of the products of the further transformation of 4 (see the next section).
Investigation of the Reactivity of N-protected Isoxazole-5-ones under Reductive Cleavage of the O− N Bond. The obtained products 4 can be particularly useful in further transformations involving the cleavage of the N−O bond which can allow the access to unprecedented compounds. During the last years, there has been an increased interest of academia and industry in molybdenum compounds in organic synthesis. 15 In particular, molybdenum hexacarbonyl Mo(CO) 6 has been used in various reactions, namely, C−C bond formation, cyclization, reductions, oxidations, and heterocyclic ring formation 15,19−25 as well as in the ring cleavage of isoxazole and isooxazoline compounds. 15,16 In particular, reductive cleavage of the O−N bond of isoxazoles and isoxazolines in the presence of the Mo(CO) 6 / H 2 O system has been used in the synthesis of new heterocycles by further in situ rearrangement of the open intermediates. 15,16 Table 1. Preliminary Investigation of the Michael Reaction of ((Chloromethyl)sulfonyl)benzene 1 with 3-Methyl-4benzylideneisoxazol-5-ones 2 However, the effect of this system has been scarcely investigated on isoxazol-5-ones derivatives. 25 After O−N cleavage of 4, in principle, the supposed formation of enamine or carboxylate groups could lead to competitive intramolecular displacements of chloride to afford five-membered heterocyclic compounds.
In order to explore the reactivity of 4 under reductive cleavage of the O−N bond with the Mo(CO) 6 /H 2 O system, a series of differently protected compounds 4 were subjected to react with Mo(CO) 6 /H 2 O under the conditions of Table 3. N-Methyl-enamine derivative 4o gave decomposition products. N-Acetyl-enamine derivative 4j did not react. Surprisingly, the N-Boc-protected 4a led smoothly to the isolation of an unprecedented β-alkylated γ-functionalized ketone 5a, which cannot be easily obtained by other methods such as 1,4conjugate additions of electron-deficient alkene 2 or by direct βfunctionalization of saturated ketones in the presence of Pd catalysts. 26 As confirmed by a series of control experiments (Scheme 2), the reaction presumably follows the order of Boc-deprotection,   Then, under the optimized conditions as reported in Table  3, the scope of the reaction was briefly analyzed with other N-Boc-protected isoxazole-5-ones 4, bearing different substituents on both the aromatic rings (Table 4). In all the cases, we obtained methylketones 5 in high yields and with a very high dr, unchanged with respect to starting materials 4. The relative configuration was determined to be (R*,R*) by comparison of experimental and calculated 1 H NMR spectra determined on 5a, 5b, and 5c. This was achieved generating conformers for each diastereomeric species using confab 27 run with an energy window of 5 kcal mol −1 . These conformers have been then reoptimized using Gaussian 16 28 at the B3LYP-gCP-D3/6-31G* scheme (see the Supporting Information for further details). 29,30 For analogy, this relative configuration can be extended to all the other ketones 5 and subsequently to Michael adducts 4.
As discussed in the Introduction section, 1,4-conjugate addition of ((chloromethyl)sulfonyl)benzenes has been exploited in cyclopropanation reactions, 1 while the isolation of the Michael adducts is quite rare. 2 Nevertheless, ketones 5 may be obtained without involving isoxazol-5-one chemistry by direct 1,4-conjugate addition of ((chloromethyl)sulfonyl)benzenes 1 to the α,β-unsaturated ketones 8 (Scheme 4). Several reaction conditions were tested as reported by Makosza et al. in the conjugated addition of ((chloromethyl)sulfonyl)benzene to nitrochalcone 2 or in accordance to the conditions described in Table 1. In all the cases, we obtained complex mixtures of unknown products, further demonstrating the utility of the approach herein described.

■ CONCLUSIONS
1,4-Conjugate additions of ((chloromethyl)sulfonyl)benzene to arylideneisoxazol-5-ones were investigated. In order to overcome the drawbacks related of the scarce stability of the obtained Michael adducts, an effective N-trapping by a sequential one-pot addition of electrophiles was developed. This strategy allowed the isolation of a wide range of new, stable isoxazole-5-ones in good yields and with high diastereomeric ratios. Then, the obtained products were subjected to the reductive cleavage of the O−N bond in the presence of the Mo(CO) 6 /H 2 O system. This further investigation led to development of an effective cascade reaction, leading to a new class of methylketones β-substituted . The 13 C NMR spectra were recorded under broad-band proton decoupling. The following abbreviations are used to indicate the multiplicity in NMR spectra: s-singlet, d-doublet, t-triplet, q-quartet, dd-doublet of doublets, m-multiplet, and brs-broad signal. Coupling constants (J) are quoted in Hertz. High-resolution mass spectroscopy (HRMS) spectra were acquired using a Bruker SolariX XR Fourier transform ion cyclotron resonance mass spectrometer (Bruker Daltonik GmbH, Bremen, Germany) equipped with a 7T refrigerated actively shielded superconducting magnet. At LMU Munchen, HRMS spectra were recorded on a Finnigan MAT 90, a Finnigan MAT 95, a Thermo Finnigan LTQ FT Ultra Fourier Transform ion cyclotron resonance, or a Q Exactive GC Orbitrap GC/MS. For ionization of the samples, either electron-impact ionization (EI) or electrospray ionization (ESI) was applied. Selected IR spectra (4i, 4m, 4o, and 5c) were recorded in KBr on a Bruker Vertex 70 spectrometer.
General Procedure for the Synthesis of Compounds 4a−4q. 4-Alkylideneisoxazol-5-ones 2 (0.107 mmol, 1.0 equiv) were added to a solution of ((chloromethyl)sulfonyl)benzenes 1 (0.128 mmol, 1.2 equiv) and potassium tertbutoxide (0.107 mmol, 12 mg, 1.0 equiv) in anhydrous CH 3 CN (0.21 M, 0.50 mL) at −20°C. The reaction mixture was monitored by TLC until complete disappearance of starting materials; after that, the reaction mixture was treated with the electrophilic trapping reagents (E−X = Boc 2 O or Ac 2 O or CH 3 I, 0.214 mmol, 2 equiv) and warmed to room temperature. The reaction mixture was allowed to stir until the disappearance of the starting materials on TLC (hexane/ethyl acetate = 80:20). The solution was evaporated, affording the crude product as a white solid, which was purified by column chromatography (hexane/ethyl acetate 80:20) to provide 4a− 4q (63−86%). The reaction with substrate 2a was scaled to 0.535 mmol (100 mg), leading to the product in 68% yield (0.364 mmol, 174 mg).